There is a growing trend in the general "case and carton sealing" as well as the "carton and tray forming" market towards high speed production so as to provide lower unit costs and higher profit levels. However, the use of such higher machine speeds often excludes the use of normal water based adhesives, such as those based on dextrins, polyvinyl acetate and latex. The packaging industry has therefore moved towards higher technology products, such as hot melt adhesives which are fast setting, easy to apply, and clean to use. These adhesives provide a fair degree of high and low temperature resistance, but are attacked by aromatic oils, solvents and petrochemical vapors.
Consequently, some food containing cartons, particularly those containing aromatic oils and mint based foods, cannot be successfully sealed with hot melts. For example, certain teas contain oil of Bergamont. After the carton is filled with tea, it is sealed with hot melt and then overwrapped with polypropylene or similar film to prevent ingress of moisture, and loss of flavour. If cartons are examined within a few hours a good bond is obtained; however, after a week the hot melt will be seen to be weakening, and eventually total bond failure will result. This failure is due to the hot melt being attacked by the oil in the tea,
A similar situation also exists in non-food applications. These are typified by soap and detergent powder essences, fire lighter fuels, and light engineering oils. Here again, the hot melt is attacked, and bond failure results.
Bonding of silane, fluorocarbon and other treated stocks is also hampered by the use of hot melt adhesives which do not "wet" the coated surface sufficiently to insure adequate adhesion. In these cases, it is necessary to "block-out", i.e. leave uncoated, those portions of the paperboard stock which will subsequently be positioned in the sealed areas.
The development of "cork wads" has also been retarded by the lack of suitable adhesives. "Wads" is the term given to the liquid seal in the cap of a bottle. They are normally constructed from an inert plastic material bonded to either naturally occurring cork or a cork derivative. A laminate is produced, which is subsequently glued into the cap. It is this latter gluing operation which is often affected by the materials or chemicals contained within the bottle or jar. These chemicals can vary from solvents, through light engine oils to liquid medications. At present, most wads are retained in the caps by water based products, which take many days to set, since there is nowhere for the water to escape.